Frequency modulated magnetrons



June 24, 1958 P. L. SPENCER 2,840,785

FREQUENCY MODULATED MAGNETRONS Filed June 29, 1954 #161! ram Amwrap P54 0 A. pz/wm xrraewsy United States Patent 2,840,785 FREQUENCY MODULATED MAGNETRONS Percy L. Spencer, Waban, Mass., assignor to Raytheon ManufacturingCompany, Waltham, Mass., a corporation of Delaware Application June 29, 1954, Serial No. 440,132 14' Claims. (Cl. 332- This invention relates to a high frequency generating device and, more particularly, to a frequency modulated grid control electron discharge device of the magnetron type. Q

As is well known in the art, a multiple-cavlty magnetron includes an anode structure consisting of a plurality. of closely coupled resonant circuits in the form of cavity resonators bounded at least partially by a pair of adjacent anode members and a cathode centrally mounted with respect to the anode and spaced therefrom. Electrons emitted from the cathode are caused to move more or less circumferentially about the cathode in response to the combined presence of an electric field applied between anode and cathode. and a transverse magnetic field produced in the magnetron interaction space normal to said electricfield. Electron interaction with the radiofrequency fields between anode segments is such as to group, the electrons into bunches which sweep past the anode gaps and interact with said radio frequency field acrosssaid gaps. High frequency energy is thus coupled into the resonator system from whence it may be extracted by appropriate means. a

A grid-controlled, multiple-cavity magnetron generator, whoseoutput is amplitude modulated in accordance with the application of a, modulating voltage between the cathode and a grid structure positioned adjacent the anode vane tips, is disclosed in a copending application of P.-.L. Spencer'for U, S.-;Letters Patent, Serial No. 233,634, new Patent 2,784,345, dated-March 5, 1957, filed June 26,1951. The grid structure-in, this amplitude modulated magnetron is. drivennegative with respect to the anode and the proximity of the bunched electron rotating about the cathode tothe tips of the. anode members or vanes is varied in response to the modulating voltage, thereby changing the alternating current voltage coupled to the cavity resonators and, consequently, modulating the poweroutpu t.

In accordance with the s'ubje ct invention the control grid, which is positioned between'vane tips of the multiplecavity magnetron, is disposed well within the cavity resonators of the magnetron anodestructure so that electrons may be pulled into the cavity resonators and elfect tuning of said magnetron. This is in contrast to the aforesaid copending application inwhich the grid elements are'set back by only a few thousandths of an inch from the vanetips' to insure that thegrid elements do not lie in the path of: the bunched electrons, thereby interfering with the production of oscillations by the interception of a substantial portion of theplate current as wellas preventing overheating of the grids. owing to electron bombardment.

The control 'grid'of the magnetron according to the ing near the tips of the anode vanes affects the dielectric constant in the resonators and, consequently, affects the magnetron operating frequency. By applying. a modulating voltage between the grid and the anodes which swings between values which are always positive with respect to the anodes, the intensity of the electron stream drawn into the cavity resonators is changed and the operating frequency of the magnetron is varied accordingly.

If the control grid is positioned too close to the anode vane tips, the grid voltage necessary to bring electrons out of the positive field at the anode vane tips and into the cavity resonators becomes excessive. If, on the other hand, the grid is set back too far from the anode vane tips, the modulation sensitivity is also reduced, in this case owing to the remoteness of the grid from the cathode. In one embodiment of invention, a setback of approximately one millimeter was used; the setback will, of course, depend upon the particular parameters of the tube under consideration.

The control grid is made of, or coated with, a material having a low secondary emission ratio, which may be defined as the ratio between thetotal number of secondary electrons emitted to the total number of primary electrons striking said emitter. If the control grid is substantially incapable of secondary emission, the number of available electrons in the cavity resonator, and hence the magnetron operating frequency, may be reliably controlled.

The various elements of the control grid structure are mounted in a common support and thus may be maintained at the same potential. This allows for identity of tuning effect in each cavity resonator of the magnetron,

as compared with directly heated elements, which may not be reproduced with exactitude and which are not stable with time.

Referring to the drawing:

Fig. 1 is a longitudinal central cross-sectional view,

partially broken away, of an embodiment of a magnetron electron discharge device, having a grid structure for frequency modulating purposes;

Fig. 2 is a transverse cross-sectional View taken along line 2-2 of Fig. 1', with the magnet omitted; and

Fig. 3 is a perspective view of aportion of the magnetron anode and grid assembly of Figs. 1 and 2.

Referring to Figs. 1 to 3, an electron discharge device 10 of the magnetron type is shown which comprises essentially a cylindrical anode structure 12, a cathode assembly 14, a transverse magnetic field producing means 16, a grid structure 18, and the necessary sources of operating potential, to be described later.

Anode structure 12 includes an electrically conductive cylinder 20 from whose inner wall a multiplicity of planar electrically conductive anode members or vanes 21 extend radially inward. Each adjacent pair of anode membersZl, together with the associated portion of the wall of anode cylinder 20, form the boundary of high Q cavity resonator 22. The inner ends of the radially disposed anode vanes or members 21 serve as anode faces for receiving electrons emitted from the cathode. It should be understood, however, that the invention is not necessarily limited to magnetrons having anodes of the subject invention is maintained positive with respect to vane type. For example, magnetron anodes of the slot type or of the hole-and-slot type are equally within the purview of this invention.

A pair of concentric metallic straps 24 and 25 are positioned in an annular recess 26, in the upper edge of anode vanes 21 adjacent the inner ends thereof. Strap 24 is electrically connected to alternate anode vanes 21,

and strap 25 interconnects intervening alternate vanes in the Well known manner, thus preventing undesirable moding in the magnetron. I

Anode 12' ishermetically sealed by means of upper and lower end plates 27 and 28, respectively, which may be soldered into the ends of anode cylinder 20. The upper end plate 27 is provided with a central opening into which a magnetic pole piece 30 is hermetically soldered. The lower end plate 28 also contains a centrally disposed opening into which is hermetically soldered a second magnetic pole piece 31. An external permanent magnet or electromagnet 33, a portion of which is shown in Fig. 1, is connected to pole pieces 30 and 31 to provide a transverse magnetic field for the magnetron.

Cathode assembly 14 is centrally disposed within magnetron and includes a cathode cylinder '35 coated with a material capable of primary electron emission. Cathode 35 is coaxial with anode cylinder and is longitudinally coextensive with anode vanes 21. Annular end shields 37 and 38, whose diameter is somewhat greater than that of the cathode cylinder 35, are attached at each end of said cylinder to minimize endwise emissionfrom the cathode.

Cathode is supported by a cylindrical hollow support rod 40, which extends through an aperture in magnetic pole piece 31 and is hermetically sealed at the outer surface to a metallic cap 41. The upper portion of cap 41 is sealed to a vitreous or ceramic sleeve 42. Sleeve 42 is sealed hermetically to a metallic sleeve 43 which, in turn, is sealed to the lower end of magnetic pole piece 31. A heater lead-in conductor 44 is inserted within a bore, not shown, in support member and is insulatedly spaced therefrom. Heater conductor 44 is brought out externally of the tube through a glass bead 45 sealed to the end portion of cathode support rod 40. The lower end portion of rod 40 may contain heat radiating fins 46 for cooling purposes. The heater leadin conductor 44 terminates atone end in a heater coil, not shown, positioned with the cathode cylinder 35 and having one end attached to the inner wall .of cylinder 35. The details of the cathode heater are similar to those shown in U. S. Letters Patent, No. 2,463,524 of P. P.

Derby, issued March 8, 1949, and need notbe described herein. a 1

Oscillations generated within tube 10 may be removed by means of a coaxial output coupling device extending into one of the cavity resonators 22. The outer conductor 47 of coaxial coupling device'SO may be screwed into a threaded aperture in anode cylinder 20, while inner conductor 48 extends into the aforesaid cavity resonator and terminates in a couplingloop 49. The output coupling device need not be of the. coaxial type'but may be any desired .type such as an iris or wave guide.

The grid structure 18, which is used to frequency modulate the output of magnetron 10, comprises a supporting ring 52'sur'rounding and spaced from cathode assembly 14. Supporting ring 52 is further supported by means of radially disposed arms 53, preferably integral with ring 52. The ends of these arms are fixed to rigid supporting posts 54, which extend downwardly through apertures in the lower. end plate 28 of anode structure 12. Posts 54 are rigidly and insulatedly mounted relative to end plate 28 by means of electrically insulated seals 55 attached to a portion of metallic cylinders 56-protruding from end plate 28 and received within said apertures in said end plate. The modulation and biasing voltages are applied to one of these supporting posts 54 which extends beyond seal 55.

Grid structure 18 further includes a plurality of grid elements 58 preferably in theform of rods, one end of which is securely attached to grid support ring 52. Grid elements 58 extend upwardly from grid supportring 52 between each pair of anode members21iandarepositioned adjacent to, but further removed from, the cathode than the tips of the anode members. The surface of grid elements 58 should be made of a material. which is a poor emitter of secondary electrons. For example, the grid elements may be rhodium plated or coated with a carbonized material which has a secondary emission ratio of the order of unity.

The high voltage supply 60 for magnetron 10 may be connected between metallic sleeve 43 and metallic cap 41 which are, respectively, at the same direct current potential as the anode and cathode. The heater supply 62 is connected between cathode cap 41 and heater lead-in conductor 44.

Modulating signals from a modulating source 64 are applied to the grid structure 18 by way of a transformer 65, whose secondary winding 67 is in series with a biasing voltage derived from variable unidirectional source 68. By varying the position of movable tap 69, the proper magnitude of the biasing voltage-which is positive with respect to the anode-may be obtained. The movable tap 69 for biasing source 68 is connected to one end of the secondary winding 67 of modulation input transformer 65.

By virtue of variationsin the potential of grid structure 18 relative to anode members 21, the number of electrons permitted to enter the associated cavity resonators is likewise altered. If, for example, the grid is made more positive relative to the anode, electrons from the space charge will be pulled into the cavity resonators in which the corresponding grid elements are disposed. As more electrons enter the resonators, the dielectric constant of the resonators is increased and the resonant frevuency of said resonators is decreased. As the beam intensity is decreased, the magnetron operating frequency is correspondingly increased. The magnetron output is therefore frequency modulated in response to variations in the voltage between the grid and anode.

The source of modulating ivoltage may be connected between the grid and the cathode, if desired, but owing to the much greater distance of the grid fromthe cathode as compared with the grid-anode spacing the modulation requirements would be considerably stricter.

This invention is not limited-to the particular details of construction, materials and processes described, as

many equivalents will suggest themselves to 'those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. An electron discharge device comprising a cathode, a signal transmission network structure, means for directing electrons emitted from said cathode along paths adjacent to saidnetwork structure, a grid'structure including a plurality of elements havinga low secondary emission ratio and positioned between portions of said signal transmission network structure, said. grid structure being maintained positive with respect to said signal transmission network structure, a modulatingsignal, and means including said grid structure responsive to said modulating signal for varying the number of electrons existing in said portions of said signal transmission network structure.

2. An electron discharge device comprising a cathode, a signal transmission network structure, means for. directing electrons emitted from said cathode along paths adjacent to said network structure, a grid structure including a plurality of elements positioned between portions of said signal transmission network structure. and maintained positive with respectto said signal transmission network structure, a modulating signal, andmeans including said grid structure responsive vto said modulating signal for varying the-number of electrons existing in said portions of said signal transmission network structure.

3. A tunable magnetron comprisinga cathode, an anode structure at least partially defining a plurality of cavity resonators, and means for varying the frequency of oscil- 'lation of said magnetron, said means including a grid structure arranged concentric with said cathode and a,

source of: modulating potential applied to said gridstructure, said grid structure including a pluralityof ,grideles.

'5 ments which are substantially incapable of electron emission, said grid structure being maintained positive with respect to said anode structure by means of saidmodulating potential.

4-. A tunable magnetron comprising a cathode, an anode structure at least partially defining a plurality of cavity resonators, and means for varying the frequency of oscillation of said magnetron, said means including a grid structure arranged concentric with said cathode and a source of modulating potential applied to said grid structure, said grid structure including a plurality of grid elements having a low secondary emission ratio and each positioned within a corresponding one of said cavity resonators, said grid structure being maintained positive with respect to said anode structure by means of said modulating potential.

5. A tunable magnetron comprising a cathode, an anode structure at least partially defining a plurality of cavity resonators, and means for varying the frequency of oscillation of said magnetron, said means including a grid structure arranged concentric with said cathode and a source of modulating potential applied to said grid structure, said grid structure including a plurality of grid elements having a low secondary emission ratio and each positioned within a corresponding one of said cavity resonators between portions of said anode structure which are nearest to said cathode, said grid elements being further removed from said cathode than the portions of said anode nearest to said cathode,.said grid structure being maintained positive with respect to said anode structure by means of said modulating potential.

6. A frequency modulating oscillating system including an electron discharge device comprising a cathode, an anode structure spaced from said cathode and at least partially defining a plurality of cavity resonators, means for producing electric and magnetic fields within said device which are transverse to one another, means responsive to said transverse fields for directing electrons from said cathode along paths adjacent to portions of said anode structure, a grid structure including a plurality of electrically interconnected grid elements positioned within corresponding ones of said cavity resonators and being further removed from said cathode than the portions of said anode nearest to said cathode, means for biasing said grid structure positively with respect to said anode structure, and means connected in series with said biasing means for superimposing a frequency modulating signal upon said grid structure.

7. A frequency modulated oscillating system including an electron discharge device comprising a cathode, an anode structure spaced from said cathode and at least partially defining a plurality of cavity resonators, means for producing electric and magnetic fields within said device which are transverse to one another, means responsive to said transverse fields for directing electrons from said cathode along paths adjacent to portions of said anode structure, a grid structure arranged concentric with said cathode and including a plurality of grid elements having a low secondary emission ratio and being further removed from said cathode than the portions of said anode nearest to said cathode, means for biasing said grid structure positively with respect to said anode structure, and means connected in series with said biasing means for superimposing a frequency modulating signal upon said grid structure.

anode structure, a grid structure arranged concentric with said cathode and including a plurality of grid elements having a low secondary emission ratio and positioned within said cavity resonators, means for biasing said grid structure positively with respect to said anode structure, and means connected in series with said biasing means for superimposing a frequency modulating signal upon said grid structure.

9. A magnetron oscillator comprising a cathode, an anode structure including a multiplicity of cavity resonators, means including transversely directed electric and magnetic fields within said device for directing electrons from said cathode along paths adjacent to portions of said anode structure and in energy coupling relationship with said cavity resonators, a grid structure including a plurality of grid elements having a secondary emission ratio of the order of unity, said grid elements being positioned within corresponding cavity resonators and maintained at a biasing potential which is positive with respect to said anode structure and said cathode, a source of modulating potential applied to said grid structure in series with said biasing potential, and means responsive to said modulating potential for varying the number of electrons entering said cavity resonators from the space between said anode structure and said cathode whereby the frequency of oscillations produced by said magnetron may be varied.

10. A magnetron oscillator comprising a cathode, an anode structure at least partially defining a plurality of cavity resonators, means for directing electrons emitted from said cathode along paths adjacent to, and in energy coupling relationship with, said anode structure, a grid structure including a plurality of electrically interconnected grid elements each having a low secondary emission ratio and each positioned within a corresponding cavity resonator, said grid structure being maintained at a potential which is positive with respect to said anode structure and said cathode, means responsive to variations in said potential for electronically adjusting the frequency of said cavity resonators.

11. A magnetron oscillator comprising a cathode, an anode structure at least partially defining a plurality of cavity resonators, means for directing electrons emitted from said cathode along paths adjacent to, and in energy coupling relationship with, said anode structure, a grid structure including a plurality of grid elements each having a low secondary emission ratio and each positioned within a corresponding cavity resonator, said grid structure being maintained at a potential which is positive with respect to said anode structure, means responsive to variations in said potential for electronically adjusting the frequency of said cavity resonators.

12. A11 electron discharge device comprising a cathode, an anode structure comprising a multiplicity of radially disposed members spaced from and coaxially arranged with respect to said cathode, each adjacent pair of said anode members together with that portion of said anode structure lying therebetween bounding a cavity resonator, means for producing an electric field between said anode structure and said cathode, means for producing a magnetic field in the region between said anode structure and said cathode which is normal to said electric field, means responsive to said electric and magnetic fields for directing electrons from said cathode along paths adjacent to portions of said anode structure, a grid structure arranged concentric with said cathode and including a plurality of electrically interconnected grid elements having a secondary emission ratio of the order of unity and each positioned within a coresponding one of said cavity resonators, said grid structure being maintained at a biasing potential which is positive with respect to said anode structure and said cathode, a source of modulating signals applied to said grid in series with said biasing potential, means for altering the potential 7 of said grid structure in response to said modulating signals, means responsive to variations in said grid potential for varying the number of electrons entering said cavity resonators and consequently varying the frequency of oscillations produced therein.

13. An electron discharge device comprising a cathode, an anode structure comprising a multiplicity of radially disposed members spaced from and coaxially arranged with respect to said cathode, each adjacent pair of said anode members together with that portion of said anode structure lying the'rebetween bounding a cavity resonator, means for producing an electric field between said anode structure and said cathode, means for producing a vmagnetic field in the region between said anode structure and said cathode which is normal to said electric field, means responsive to said electric and magnetic fields for directing electrons from said cathode along paths adjacent to portions of said anode structure, a grid structure arranged concentric with said cathode and including a plurality of grid elements each positioned within a corresponding one of said cavity resonators, said grid structure being 'maintainedat a biasing potential which is positive with respect to said anode structure, a source of modulating signals applied to said grid in series with said biasing potential, means for altering the potential of said grid structure in response to said modulating signals, means responsive to variations in said grid potential for varying the number of electrons entering said cavity resonators and consequently varying the frequency of oscillations produced therein.

14. An electron discharge device comprising a cathode, an anode structure comprising a multiplicity of radially disposed members spaced from and coaxially arranged with respect to said cathode, each adjacent pair of said anode members'together with that portion of said anode structure 'lying therebetween at least partiallydefining a cavity resonator, means for producing an electric field and a magnetic field transverse to said electric field within said device, means responsive to said electric and nagnetic'fields for directing electrons from said cathode along paths adjacent to portions of said anode structure, a grid structure arranged concentric with said cathode and including a plurality of electrically interconneclcd cold grid elements having a low secondary emission ratio, said grid elements each positioned within a region bounded by adjacent pairs of said anode members and further removed from said cathode than the portions of said anode nearest to said cathode, said grid structure elements being maintained at a biasing potential which is positive with respect to said anode structure and said cathode, said grid structure being electrically insulatedly supported from said anode structure, a source of modulating potential applied to said grid in series with said potential, means responsive to said modulating potential for varying the number of electrons entering said cavity resonators, and means responsive to the number of electrons within said resonators for varying the frequency of oscillations produced by said device.

References Cited in the file of this patent UNITED STATES PATENTS 2,217,745 Hausell Oct. 15, 1940 2,468,127 Smith Apr. 26, 1949 2,527,699 Bowen Oct.'3l, 1950 2,530,185 Steele Nov. 14, 1950 2,750,506 Haagemsern June 12, 1956 

